When transmitting communication signals between a master or transmitting station and a slave or receiving station, communication signals present in analog form must frequently be transmitted in digital form over the transmission circuits connecting the master station with the slave station. In the process, the communication signals concerned are frequently transmitted in pulse code modulated (PCM) form. However, they are frequently first available in pulse amplitude modulated (PAM) form at the sending end. And after transmission to the slave station as PCM signals, they must often be forwarded again at the receiving end as PAM signals.
In connection with the conversion of PAM signals into PCM signals, it is old in the art (West German Unexamined Pat. No. 1,913,132) to compare the PAM signal with a uniformly varying reference voltage, i.e., a sawtooth voltage and, in case of similarity, to establish a criterion whereby a counter governed by clock signals is stopped at the start of the comparison; the counter position defines the PCM signal representing the PAM signal being converted. In the above context, it is likewise old to use the PCM signal for the conversion of PCM signals into analog signals to control a sawtooth generator to supply a signal corresponding in amplitude to the bit combination thereof. For this purpose, a counter is accordingly set by means of the PCM signal concerned after its line-to-store transfer in a shift register, said counter being operated such that its counter position is restored to zero. When the home position is reached, a disconnect signal is delivered to a scanning and storage circuit which stores the instantaneous amplitude of a sawtooth voltage supplied by said sawtooth generator; the sawtooth generator concerned starts supplying the sawtooth voltage at the moment when the counter starts counting down. Although analog signals can be converted into PCM signals and PCM signals into analog signals by means of the prior art arrangements referenced above, a fairly large amount of circuitry is necessary nevertheless. Moreover, these prior art arrangements are only capable of causing a linear conversion of analog signals into PCM signals and of PCM signals into analog signals, which is often undesirable.
A further digital-to-analog converter has become known (West German Unexamined Pat. No. 1,938,690), which also employs a sawtooth generator for the purpose of converting a digitally encoded signal into an analog signal. This sawtooth generator starts delivering a sawtooth voltage at the moment when a counter is caused to take successively different positions to which corresponding signals representing the digitally encoded signal being converted are compared in a comparator. If this comparator detects correspondence between the digitally encoded signal being converted and the signal corresponding to a position of said counter, it actuates a switching device connecting the outlet of the sawtooth generator with a storage capacitor on which, consequently, the instantaneous amplitude of the sawtooth voltage available at the closing instant of the switching device is stored. This prior art converter, too, causes only a linear conversion of the digitally encoded signal concerned into an analog signal. Likewise, no measures whatever are taken in this case to convert a digitally encoded signal into an analog signal which take into account a non-linear characteristic.
As mentioned hereinbelow, a conversion of analog signals into PCM signals and a conversion of PCM signals into analog signals is frequently desirable, wherein account may be taken of a non-linear characteristic or a bend characteristic. To carry out said conversion, a commonly used technique (West German Examined Patent Application No. 1,537,970) is to employ a coder used for PCM transmission systems operating according to the feedback principle and having a non-linear companding characteristic with multiple bends, and a decoder having a non-linear companding characteristic with multiple bends. The coder is so constructed that an analog signal being converted into a PCM signal is fed into one input of a comparator, a varying reference voltage supplied by a voltage source being fed into another input. The voltage source is triggered from a clock-actuated logic element over a code converter to supply a voltage which varies according to a desired course, that is to say, along the course of a non-linear companding characteristic with multiple bends. If the comparator detects a correspondence between the analog signal fed thereinto and the voltage supplied by said voltage source, it delivers a signal, in reaction to which a code combination is provided by the logic element and the companded PCM signal corresponding to the analog signal concerned.
In the decoder, the PCM signal, after being received by a shift register and forwarded by transfer gate circuits and by a 7-bit storage, is fed into a code converter, which triggers at its outgoing end a voltage source corresponding to the above voltage source, from which the analog signal corresponding to the PCM signal concerned (but presently expanded again) is delivered at the outgoing end. Such voltage sources are particularly formed by resistor ladder networks which are triggered by constant currents.
Although the last-mentioned prior art coder permits a conversion of analog signals into PCM signals, taking account of a non-linear bend characteristic, and the last-mentioned decoder permits a conversion of PCM signals into analog signals, taking into account a non-linear bend characteristic, a fairly large amount of circuitry is still necessary. This is particularly true of the voltage sources and code converters to be separately provided in the coder and the decoder.